St. Croix Crossing designed, built to protect natural environment
Bridge ensures area’s integrity through preservation, restoration, and mitigation
STILLWATER, Minn. (June 27, 2017) – Prior to allowing traffic to drive across the St. Croix Crossing, architects, engineers, contractors and workers elevated environmental compatibility and protection into every aspect of the project.
The bridge designers and builders faced challenges at every stage of the project. The bridge on the project lies within a national scenic riverway and adjacent to a historic area, and the area is inhabited by threatened and endangered species. Every challenge amounted to an opportunity to meet or exceed established rules for protecting sensitive ecosystems.
“From the very beginning, we have worked to consider the environmental impact of this project,” said Terry Zoller, St. Croix Crossing construction manager. “We’ve done all we can to avoid environmental impact, and have planned and executed the most extensive mitigation package in MnDOT history to offset those impacts.”
Architectural design and engineering
During the bridge’s planning and design phase, stakeholders chose an extradosed bridge structure to reduce the visual and environmental impact of the bridge. This type of bridge, which combines the benefits from both the boxed-girder bridge and the cable support design, reduced the total number of piers required in the waterway, allowed for a lower tower height, and provided a more elegant look with elements such as the stay cables attached to the bridge deck.
“With the extradosed design, we were able to make the bridge structure slim and reed-like, enhancing the natural beauty of the landscape instead of becoming the focal point of it,” said Todd Clarkowski, St. Croix Crossing project coordinator.
Bridge developers surveyed ravines during the planning phase and aligned the bridge with an existing ravine on the Wisconsin side. This reduced the need cut into the bluff and created a smooth transition from the highway to the bridge. Tan was chosen as the bridge paint color so it would mirror the hue of the river bluffs. And the St. Croix Crossing’s lighting system will use directional instruments that ensure a safe driving surface while minimizing nighttime light spillover into the river valley.
The bridge’s drainage system was carefully designed to meet or exceed stormwater quality requirements. The system features 16 ponds which filter stormwater—10 in Minnesota and 6 in Wisconsin. The bridge’s stormwater system carries rainwater and snowmelt directly to the ponds to be treated before being released back into the river. By filtering out materials such as sand, sediment, gravel and nutrients, the holding ponds will benefit the water quality and reduce phosphorus by approximately 23% in the stormwater. “The water coming out of the new bridge’s holding ponds will be cleaner than the water currently running off the Lift Bridge into the St. Croix River,” said Clarkowski.
Federally protected and endangered species also presented unique challenges for bridge developers. For example, the interchange at Highway 36 and Highway 95 was designed to avoid impact to a bald eagle nesting tree. In addition, Higgins eye pearly mussels living in the bridge construction area were relocated prior to construction, and the U.S. Fish and Wildlife Service was compensated to raise new mussels at an off-site facility. These native mussels will eventually be transplanted into the St. Croix River and other tributaries. In Wisconsin, seeds of endangered Dotted Blazing Star flowers were gathered from existing flowers and stored for future planting before crews relocated the flowers.
Additionally, historic properties were assessed and mitigated appropriately. For example, the Shoddy Mill and Warehouse were preserved and relocated north along Highway 95 in Stillwater.
During construction, crews diligently worked to protect water quality. Frequent, routine water measurements during key phases of construction determined pH levels and required water to be at a safe, neutral level before being released back into the St. Croix River. MnDOT and contractors collaborated with private citizens who have monitored water quality for decades. They shared data to gain the most accurate measurements. Crews also built a water treatment plant and brought water containing construction sediment there to be treated. Turbidity curtains temporarily controlled soils within work zones from migrating downstream or to other areas, and concrete barriers and silt fencing guarded wetland edges from construction traffic.
When working near the Wisconsin bluffs, construction crews used less invasive construction techniques that included constructing the drainage structure by hand, operating small machines and equipment, and using a temporary trestle system that reduced impact to the bluffs below Pier 13 at the east abutment.
At the barge unloader facility near the bridge, crews prevented the disturbance of an active bald eagle nest by keeping a strict 300-foot perimeter. This spring, three eaglets were observed in the nest instead of the normal one hatchling. In addition, managers and crews took steps to prevent the introduction of invasive species into the river ecosystem.
Crews also removed the King Plant mooring cells from the St. Croix River, spreading the aggregate from inside the cells along the riverbed to provide aquatic habitat for fish spawning and mussels. Construction activities also met or exceeded requirements regarding noise, vibration, and air quality.
Long after the St. Croix Crossing opens to traffic in late summer, the bridge’s environmental measures will continue to preserve and improve the pristine quality of the surrounding natural environment.
In 2019, when the Stillwater Lift Bridge is reopened to the public after extensive renovation, the five-mile Loop Trail will connect the St. Croix Crossing with the Lift Bridge, enabling hikers and bikers to safely travel from one bridge to the other and back again, on both sides of the unique river ecosystem. Currently open, the scenic overlook on the bluff in Oak Parks Heights offers an outstanding vantage point to observe the new bridge. As part of the project, the scenic overlook was restored to its 1929 appearance.
“The St. Croix Crossing bridge is a great example of how stakeholders and departments of transportation can respect the environment and still meet transportation needs for generations to come,” said Zoller.